1. Field of the Invention
The present invention relates to a reflective liquid crystal display.
2. Description of Related Art
A reflective liquid crystal display does not require a backlight as a light source, since it is so configured that an external incident light is reflected by a reflector plate provided in an inside of the liquid crystal display, and the reflected light is utilized as a light source for display. This has been considered to be an effective means capable of reducing a consumed electric power, of thinning the display and of lightening the display, as compared with a transparent type liquid crystal display. A reflective liquid crystal commercially available at present is of a direct matrix drive STN (super twisted nematic) type.
However, the STN type reflective liquid crystal display does not have a satisfactory display characteristics in connection with a brightness and a resolution. Therefore, there has been considered an active matrix type configured to drive, by means of switching means such as thin film transistors or diodes, a liquid crystal of a TN (twisted nematic) type, a GH (guest-host) type, or a PDLC (polymer dispersed liquid crystal) type. In all these conventional reflective liquid crystal displays, a reflector is provided on an insulative plate located at a side opposite to an eye viewing side, and a transparent electrode is provided on an insulative plate at the eye viewing side. In addition, a convex-concave is formed at a reflecting surface of the reflector. With formation of the convex-concave reflecting surface, a light injecting onto the reflector is scattered, so that it is possible to prevent a face of a user and a background of the user from being reflected in a screen surface of the liquid crystal display.
Furthermore, the display performance greatly varies dependently upon the position of the reflector. In the STN type and the TN type which require a polarizer, since a polarizer has to be adhered on an outside surface of each of two insulative plates, the reflector is provided on an outside of the polarizer. As a result, a separation on the order of 0.2 mm to 1.1 mm corresponding to the thickness of the insulative plate, inevitably occurs between the reflector and an image displayed by the liquid crystal, so that a double image occurs, and therefore, when characters are displayed, a fuzziness of the displayed characters occurs. On the other hand, in the GH type and the PDLC type which require no polarize, since the reflector is provided in an inside of the two insulative plates, it is possible to prevent the double image.
Referring to FIG. 1, there is shown a diagrammatic sectional view of a conventional GH type reflective liquid crystal display having the above mentioned structure.
As shown in FIG. 1, on a lower insulative plate 1, a switching device for an active matrix drive is formed, which is for example a thin film transistor (TFT) composed of a pair of source/drain electrodes 3 formed on the insulative plate 1, a doped layer 4 formed on an inner side of each of the source/drain electrodes 3, a semiconductor layer 5 formed on the insulative plate 1 between the pair of source/drain electrodes 3 and on each of doped layer 4, a gate insulator film 6 formed on the semiconductor layer 5, and a gate electrode 7 formed on the gate insulator film 6.
Furthermore, a polyimide insulator film 15 is formed to cover the switching device as mentioned above and to cover the remaining portion of the lower insulative plate 1. This insulator film 15 has an convex-concave surface, in a region other than the switching device, and a pixel electrode 8 is formed on the convex-concave surface of the insulator film 15, and therefore, the pixel electrode 8 has a convex-concave surface corresponding to the convex-concave surface of the insulator film 15. The pixel electrode 8 is connected to one source/drain electrode 3 of the switching device through a contact hole 6 formed in the insulator film 15.
With the above mentioned structure, the pixel electrode 8 functions as a reflector having a convex-concave reflecting surface.
On an upper insulative plate 2, a common electrode 9 which is a transparent electrode, is formed, and a liquid crystal material layer 10 is sandwiched between the lower insulative plate 1 and the upper insulative plate 2. An image is viewed from a side of the upper insulative plate 2.
The convex-concave of the insulative film 15 is formed by a conventional photolithography or an exposure-and-etching process using a photosensitive insulative material. This technology is disclosed by for example, (1) Japanese Patent Publication No. JP-B-61-6390, (2) Tohru KOIZUMI and Tatsuo UCHIDA, "Reflective Multicolor LCD (II): Improvement in the Brightness", Proceedings of the SID, Vol. 29/2, pp.157-160, 1988, (3) S. Mitsui et al, "23.6: Late-News Paper: Bright Reflective Multicolor LCDs Addressed by a-Si TFTs", SID 92 DIGEST, pp.437-440, 1992, and (4) Naofumi KIMURA et al, "Development of Reflective Multicolor LCD", Sharp Technical Report, No. 56, pp.27-30. June 1993. The disclosure of these publications is incorporated by reference in their entirety into this application.
As mentioned above, the conventional reflective liquid crystal display has been configured to form a convex-concave at the reflecting surface of the reflector, in order to scatter the incident light, thereby to prevent a user's face and its background from being reflected in the display screen of the liquid crystal display. However, in the case of forming a convex-concave on a plate on which an active device such as a TFT device or an MIM device is formed, it is necessary to deposit an insulating film covering the active device and to pattern the deposited insulating film so as to form a convex-concave surface. But, in the patterning process for forming the convex-concave surface, a fine control of a shape such as an inclined angle of the convex-concave is difficult, with the result that a sufficient light scattering cannot be obtained.